It is conventional in some nuclear reactors for bundles of fuel to pass through the reactor within horizontal pressure tubes. Each pressure tube is surrounded by a calandria tube. Garter springs provide a support structure for a pressure tube within a calandria tube and act to prevent contact between pressure tube and calandria tube. In an attempt to detect garter springs, it is known to place a measurement device into the pressure tube to record data related to the diameter of the pressure tube at each of a plurality of angular offsets as a function of distance along the pressure tube.